Electronic devices are typically used with power conversion circuits. Switched mode power converters are commonly used due to their high efficiency, small size and low weight to power many of today's electronics. Conventional wall sockets provide a high voltage alternating current. In a switching power converter a high voltage alternating current (ac) input is converted to provide a well regulated direct current (dc) output. In operation, a switch, included in the power converter, is utilized to control the desired output current by varying the duty cycle (typically the ratio of the on time of the switch to the total switching period) and/or varying the switching frequency (the number of switching events per unit time). More specifically, a power converter controller may determine the duty factor and/or switching frequency of the switch in response to a measured input voltage and a measured output voltage.
In one type of dimming for lighting applications, a dimmer circuit typically blocks a portion of the ac input voltage to limit the amount of power supplied to an incandescent lamp. Particularly, the dimmer circuit outputs a dimmer output voltage that is representative of an ac input voltage with portions of the ac input voltage removed or blocked to provide a dimming function. The removing or blocking of a portion of the ac input voltage may be referred to as phase dimming because it is often convenient to designate the position of the missing voltage of the dimming output voltage in terms of a fraction of the period of the ac input voltage measured in degrees. In general, the ac input voltage is a sinusoidal waveform and the period of the ac input voltage is referred to as a full line cycle. As such, half the period of the ac input voltage is referred to as a half line cycle. An entire period has 360 degrees, and a half line cycle has 180 degrees. Typically, the phase angle is a measure of how many degrees (from a reference of zero degrees) of each half line cycle the dimmer circuit blocks. As such, removal of half the ac input voltage in a half line cycle by the dimmer circuit corresponds to a phase angle of 90 degrees.
LED (light emitting diode) loads require a regulated power converter to provide regulated current from the ac power line. A controller included in the conventional regulated power converter may measure the input voltage in order to regulate the output of the power converter. Generally, a common way to measure the input voltage is to sense a peak of the ac input voltage. In certain configurations, a dimmer circuit maybe coupled between an ac source providing the ac input voltage and the sense circuit that may be used to sense the peak input voltage. Therefore, the power converter may rely on sensing an output of the dimmer circuit, the dimmer output voltage, which may still ‘capture’ or ‘hold’ the peak of the ac input voltage. Thus, natural dimming of the LED load may occur when the peak of the dimmer output voltage equals a corresponding peak input voltage of the ac power line. This is because the controller may not realize that a portion of the ac input voltage has been blocked by a dimmer circuit, thereby reducing the amount of power delivered to the output. However, when a phase angle is greater than 90 degrees, the dimmer circuit may block the peak input voltage and therefore misrepresent the peak input voltage with a lower voltage. In this situation, a controller included in the conventional regulated power converter is likely to prevent natural dimming because the controller is receiving a peak dimmer output voltage that should be representative of the peak input voltage but is instead representative of a voltage less than the peak ac input voltage. To further explain, a controller included in a conventional regulated power converter may sense the peak dimmer output voltage that is the peak dimmer output voltage that corresponds to a voltage along the ac line that is less than the peak ac input voltage and thus detects less incoming power at the input of the power converter. The controller may then increase duty cycle and/or switching frequency and inadvertently increase power delivery to the output. As a result a controller included in a conventional regulated power converter may actually prevent or ‘fight’ against natural dimming of the led load.